Such a skin treatment device is, e.g., known from the published international patent application WO 2008/001284 A2. Said application discloses a skin treatment device with a laser source and focusing optics. The device comprises a housing with an exit window through which a laser beam is emitted. The exit window may be made of a transparent material or may simply be an aperture in the housing. The device creates a focal spot in a dermis layer of the skin to be treated. The power of the laser beam and the dimensions of the focal spot are selected such that laser induced optical breakdown (LIOB) is induced in the focal spot in the skin in order to stimulate re-growth of skin tissue and to reduce wrinkles.
The focal spot is created at a fixed treatment depth, somewhere between 0.2 and 2.0 mm. This depth is selected based on the typical composition of human skin. In some cases, however, the optimal treatment depth may be different. The optimal treatment depth depends on, e.g., the thickness of the stratum corneum and the epidermis. To prevent a lens effect by the skin and by the curvature of the wrinkle, an index-matching material may be applied to the skin in order to fill the wrinkle between the epidermis and the exit window. The index-matching material should have an index of refraction approximately between the index of refraction of the exit window and that of the epidermis, preferably approximately equal to the refractive index of the epidermis. In the ideal case, the exit window and the index-matching material have the same index of refraction as the epidermis, i.e. roughly 1.4.
However, the mere addition of an index-matching material in order to prevent a mismatch in the refractive index is not sufficient for obtaining the desired control over the light intensity in the focal spot and over the exact position of the focal spot in the skin. The skin surface (stratum corneum) shows microscopic fluctuations in roughness due to vertically stacked corneocytes. The coupling of light through the optically rough stratum corneum, which resembles an array of micro lenses, is difficult and results in loosely focused light beams. This causes a decrease of the ratio of the photon density in the focal spot to the background and a less efficient LIOB creation in the focus.